The present disclosure is directed to generally to a frequency synthesizer. Several types of frequency synthesizers are known. The first and second types are stand-alone architecture direct analog frequency synthesizers (also called a mix-filter-divide architecture) and the more modern direct digital synthesizers (DDS) (table-look-up). The third type is referred to an indirect digital phase-locked loop (PLL) synthesizers, which include integer-N and fractional-N counters. The indirect digital PLL synthesizer receives a reference oscillator input and a digital frequency control word and sets the output frequency.
A direct frequency synthesizer has multiple fixed-frequency sources feeding into a switch, which outputs a desired frequency. A direct frequency synthesizer may use a PLL and a steering current circuit to achieve output frequency lock. Although a direct frequency synthesizer exhibits good rapid frequency switching performance, direct frequency synthesizers can become large and complex for a range of output frequencies. In addition, the frequency output signal can be spurious due to multiple signal sources running continuously. A frequency mixer is a type of direct frequency synthesizer, and it improves upon signal stability. It receives signals from different sources, such as the switch output of a direct frequency synthesizer and the output of a DDS, and combines them in a frequency mixer to produce an output having components of both input signals. A band-pass filter can then be used to select the desired frequency from the mixer. These devices are good for producing pure signals that are stable, and are able to change frequencies rapidly. However, they are large, complex, and expensive.
There are different methods of generating a signal of known output frequency with an indirect PLL frequency synthesizer. A digital-to-analog converter (DAC) can be used to receive a digital frequency control word and output an analog voltage signal to the tuning port of a voltage controlled oscillator (VCO). Also known as a digitally tuned oscillator, this circuit exhibits good performance for fast frequency tuning but lacks frequency stability, both long term and short term frequency stability. The present disclosure provides various signal generator circuits including DAC-assisted frequency synthesizers that provide for low noise, fast switching, and stable frequency and phase performance.